Differential imaging with pattern recognition for process automation of cross sectioning applications

ABSTRACT

A method for using differential imaging for applications involving TEM samples by allowing operators to take multiple images during a procedure involving a focused ion beam procedure and overlaying the multiple images to create a differential image that clearly shows the differences between milling steps. The methods also involve generating real-time images of the area being milled and using the overlays of the differential images to show small changes in each image, and thus highlight the ion beam milling location. The methods also involve automating the process of creating differential images and using them to automatically mill subsequent slices.

This application claims priority to U.S. Prov. Application 61/897,052filed Oct. 29, 2013 which is hereby incorporated by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to charged particle beam systems, such asfocused ion beam systems.

BACKGROUND OF THE INVENTION

Transmission electron microscopes (TEMs) allow observers to seeextremely small features, on the order of nanometers. In contrast toscanning electron microscopes (SEMs), which only image the surface of amaterial, TEMs also allow analysis of the internal structure of asample. In a TEM, a broad beam impacts the sample and electrons that aretransmitted through the sample are focused to form an image of thesample. The sample must be sufficiently thin to allow many of theelectrons in the primary beam to travel though the sample and exit onthe opposite side. Samples, also referred to as lamellae, are typicallyless than 100 nm thick.

In a scanning transmission electron microscope (STEM), a primaryelectron beam is focused to a fine spot, and the spot is scanned acrossthe sample surface. Electrons that are transmitted through the workpiece are collected by an electron detector on the far side of thesample, and the intensity of each point on the image corresponds to thenumber of electrons collected as the primary beam impacts acorresponding point on the surface. The term “TEM” as used herein refersto a TEM or a STEM and references to preparing a sample for a TEM are tobe understood to also include preparing a sample for viewing on a STEM.The term “S/TEM” as used herein also refers to both TEM and STEM.

Focused Ion Beam (FIB) microscope systems produce a narrow, focused beamof charged particles, and scan this beam across a specimen in a rasterfashion, similar to a cathode ray tube. Unlike the SEM, whose chargedparticles are negatively charged electrons, FIB systems use chargedatoms, hereinafter referred to as ions, to produce their beams. Theseions are, in general, positively charged.

Removing material from a substrate to form microscopic or nanoscopicstructures is referred to as micromachining, milling, or etching. Whenan ion beam is directed onto a semiconductor sample, it will ejectsecondary electrons, secondary ions (i+ or i−), and neutral moleculesand atoms from the exposed surface of the sample. By moving the beamacross the sample and controlling various beam parameters such as beamcurrent, spot size, pixel spacing, and dwell time, the FIB can beoperated as an “atomic scale milling machine,” for selectively removingmaterials wherever the beam is placed. The dose, or amount of ionsstriking the sample surface, is generally a function of the beamcurrent, duration of scan, and the area scanned. The ejected particlescan be sensed by detectors, and then by correlating this sensed datawith the known beam position as the incident beam interacts with thesample, an image can be produced and displayed for the operator.Determining when to stop processing is referred to as “endpointing.”While there are several known methods for detecting when amicromachining process cuts through a first material to expose a secondmaterial, it is typical to stop laser processing before a change inmaterial is reached, and so determining the end point is more difficult.

FIB systems are used to perform microsurgery operations for executingdesign verification or to troubleshoot failed designs. This usuallyinvolves “cutting” metal lines or selectively depositing metallic linesfor shorting conductors together. FIB “rapid prototyping” is frequentlyreferred to as “FIB device modification”, “circuit editing” or“microsurgery.” Due to its speed and usefulness, FIB microsurgery hasbecome crucial to achieving the rapid time-to-market targets required inthe competitive semiconductor industry.

Successful use of this tool relies on the precise control of the millingprocess. Current integrated circuits have multiple alternating layers ofconducting material and insulating dielectrics, with many layerscontaining patterned areas. The milling rate and effects of ion beammilling can vary vastly across the device. This is the reason whyendpointing is difficult to perform without it being destructive.Endpointing is generally done based on operator assessment of image orgraphical information displayed on a user interface display of the FIBsystem. In most device modification operations, it is preferable to haltthe milling process as soon as a particular layer is exposed. Impreciseendpointing can lead to erroneous analysis of the modified device.

As semiconductor device features continue to decrease in size fromsub-micron to below 100 nm, it has become necessary to mill smaller andhigher aspect ratios with ion beam currents. FIB operators rely onconventional methods using real-time images of the area being milled anda graphical data plot in real time, to determine proper endpointdetection. Generally, the FIB operator is visually looking for changesin brightness of the milled area to qualitatively determine endpointdetection. Such changes may indicate a transition of the mill throughdifferent materials, such as a metal/oxide interface. The operator usesthe progression slice to slice and looks for changes that ultimatelytell the operator where the milling is taking place, the changes in thesample, and the progression towards endpointing.

Modern techniques sometimes involve the use of dual beam systems, suchas an a FIB and SEM combination systems that allow the user to slicethrough samples and create images of the sections “live,” such asSPI—(simultaneous patterning and imaging mode), for real-time imagingfeedback on the milling processes. TEM sample prep endpointing is adecision made in real time and it can be used in cross section patterns,but the sample is generally sliced in a manner that is also destructive.In addition, SPI images often create lower resolution due to the frameaveraging of the images and the high image e-beam currents. The I-SPI isa system that allows images to update between various slices. Theseimages are refreshed at every slice, but because consecutive slicesinvolve only slight changes between images, the user often finds theseimage slices very difficult to follow.

There are generally two different ways to collect a stack of 2D SEMimages of FIB milled surfaces for subsequent 3D modeling of volumesusing a dual platform FIB/SEM instrument, i.e., in static or dynamicmode. In dynamic SEM imaging of FIB milled surfaces (i.e., SPI mode),SEM images are acquired in real time during the FIB milling process. Instatic image acquisition mode, the FIB is used to slice away materialand then either paused or stopped so that a slow scan high resolutionSEM image may be acquired. This type of image acquisition can be easilyprogrammed into an automated Slice and View algorithm or an intermittentor I-SPI mode of instrument operation.

In SPI mode, secondary electrons (SEs) are emitted and detected due toion/solid interactions as well as electron/solid interactions. To swampout the SE signal from the FIB milling in the SEM image, the SEM imageacquisition must be performed by changing three critical SEM imagingparameters: (i) the SEM beam current must operate at approximately afactor of 2 or greater than the ion beam milling current, (ii) the SEMimages must be acquired at very fast scan rates, and (iii) the SEMimages must be acquired using frame averaging (e.g., as many as 32 or 64frames may be required for large beam current milling). Thus, SE SEMacquisition of images obtained in SPI mode must be collected in a modewhich is typically not used for highest resolution imaging.Alternatively, backscattered electron (BSE) SEM images can be collectedin SPI mode where the SEs from the FIB milling produces negligibleartifacts in the BSE imaging process. However, the timing of imageacquisition during SPI mode is critical, and even the acquisition of BSESEM images in SPI mode may be non-trivial.

SEM images acquired in SPI mode can obtain redundant and/or duplicateinformation from one or more slices. Thus, using SPI mode, one wouldhave to manually search through the sequence of images to removeredundant images such that an accurate 3D model could be constructed.One could time each SE or BSE image saved such that it occurs only aftera complete FIB slice, but this would require a prior knowledge of thematerial sputtering characteristics and would be difficult to exactlycorrelate the SEM acquisition time with the time needed to FIB through aslice. It is noted however that SPI mode is extremely useful forendpointing any FIB operation since FIB milling may be monitored in realtime.

The advantage to the static Slice and View methods for 3D modeling isthat a high resolution slow scan SEM image is acquired after each FIBmilled slice is completed. Thus, each image corresponds uniquely to eachFIB slice for easy volume determination. In addition, automated SEM beamshift and auto-focus corrections can be implemented to keep the regionof interest centered and focused as sectioning progresses.

Prior art methods have tried to improve on FIB milling endpointingoperations by generally creating a real-time ability to gauge thesensitivity to regions of interest on a sample site. For example, EP1812946 A1, with a filing date of Nov. 15, 2005 (also published as U.S.Pat. No. 7,897,918), titled “System and method for focused ion beamdata,” (hereinafter as the '918 patent) discloses a system and methodfor improving FIB milling endpointing operations by using real-timegraphical plots of pixel intensities with an increased sensitivity overnative FIB system generated images and plots. This is done by receivingdwell point intensity values and creating raster pattern data to createareas of sensitivity. As shown in FIG. 9 of the '918 patent disclosed amethod of using snapshot images taken progressively in a raster pattern.The frame generation is done using a CPB system. And as shown, thedifferential images 428, 430 are made using the individual slices.

This method has many shortcomings, which should be apparent. Theaccuracy and timelines of this procedure does not allow the operatorfull control of the endpointing nor would it allow the operator to seeclear distinctions in patterns or defects. Further, the time consumptionfor this procedure would not allow the operator to perform this functionin a manner that can enable real-time endpointing with concurrent use ofprogressive endpointing.

With current technologies that can be used to manipulate endpointing,there are generally two modes for collecting a series of SEM images ofFIB-milled surfaces using a dual beam instrument: static mode anddynamic mode. In dynamic SEM imaging of FIB milled surfaces, SEM imagesare acquired in real time during the FIB milling process. In staticimage acquisition mode, the FIB is used to slice away material and theneither paused or stopped so that a slow scan high resolution SEM imagemay be acquired.

One challenge during dynamic mode imaging, is to know exactly where thebeam is hitting the sample, and what part of the sample is being milledat any given moment. Particularly in the preparation of a TEM sample,the user must determine in real time when to stop thinning a sample.Thinning the sample too much can destroy it. The lower resolution ofdynamic mode can make it difficult to know how much the sample has beenthinned.

Thus, the user typically watches the progression from slice to slice andlooks for changes from slice to slice. This helps him know where themilling is taking place (where the beam “is hitting”), helps him seechanges in his sample, and helps him follow progress towards theendpoint—where the user manually stops the mill, e.g., endpointing.

What is needed is a method of precisely and efficiently showing theoperator changes in the slices so that real-time endpointing can be donein a better manner that produces less errors and higher productivity.The method lends itself further with other automated processes thatincrease throughput and reproducibility of TEM samples.

SUMMARY OF THE INVENTION

An object of the invention, therefore, is to provide an improved methodfor endpointing that allows the operator to view the samples in a mannerthat can visually distinguish slices. Preferred embodiments of thepresent invention use overlays of differential images to further improveI-SPI modes and provide improved methods for endpointing samples thattogether increases throughput and reproducibility of TEM samplecreation.

Another object of the invention is to provide an improved method forimproved yield, speed, and accuracy when performing endpointingprocedures. Preferred embodiments of the present invention use overlaysof differential images to better distinguish differences between slicesthat improve the visual contract of endpointing results that lead toimproved yield, speed, and accuracy. By using overlays, there is thepotential to use successive overlays that will create a pipeline ofimages, which allows the operator to process the first set of imageswhile the pipeline of successive images is being processed. This allowsfor faster throughput and higher efficiency.

Another object of the invention is to provide an endpointing procedurethat can be automated. Preferred embodiments of the present inventionallow systems to use overlays that automatically show differences inoverlays of differential images.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter. It should be appreciated by those skilled in the art thatthe conception and specific embodiments disclosed may be readilyutilized as a basis for modifying or designing other structures forcarrying out the same purposes of the present invention. It should alsobe realized by those skilled in the art that such equivalentconstructions do not depart from the spirit and scope of the inventionas set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more thorough understanding of the present invention, andadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagram of side by side images of TEM sample slices usingconventional methods;

FIG. 2 shows a diagram showing TEM sample preparations according to thecurrent invention wherein the diagram represents slices of TEM samplesthat are overlayed;

FIG. 3A is a sample TEM slice labeled “n”;

FIG. 3B is the next sample TEM slice labeled “n+1”;

FIG. 3C is the differential image of FIGS. 3A and 3B;

FIG. 3D is the differential image of FIGS. 3A and 3B that is processed;

FIG. 4A is a cross sectional image of a copper grain TEM sample;

FIG. 4B is another cross sectional image of a copper grain TEM sample inthe series;

FIG. 4C is the differential of the cross sectional images of coppergrain TEM samples;

FIG. 5A is a TEM sample 22 nm plan view having slice n;

FIG. 5B is a TEM sample 22 nm plan view having slice n+1;

FIG. 5C is the overlay process view with slice n+1 of FIGS. 5A and 5B;

FIG. 5D is the overlayed differential view according to an embodiment ofthe current invention;

FIG. 5E is a TEM sample of a 22 nm plan view having the label slice n+2;

FIG. 5F is a TEM sample of a 22 nm plan view having the label slice n+3;

FIG. 5G is the overlayed differential view according to an embodiment ofthe current invention of FIGS. 5A-5F;

FIG. 5H is the overlayed image showing the slice n+3;

FIG. 5I is the differential image showing slice n+3;

FIG. 5J is a flashing overlay showing slice n+3 in accordance with thecurrent invention;

FIG. 6 is a flowchart 600 showing an exemplary method of usingdifferential imaging to perform endpointing in a charged particle beamsystem in accordance with embodiments of the present invention;

FIG. 7 depicts of an exemplary dual beam SEM/FIB system 702 that isequipped to form samples and move them to a TEM grid;

FIG. 8 is a micrograph image showing a sample before a millingoperation;

FIG. 9 is a micrograph image showing the sample after a millingoperation;

FIG. 10 is an image formed by subtracting the image of FIG. 8 from theimage of FIG. 9;

FIG. 11 is an image formed by subtracting the image of FIG. 9 from theimage of FIG. 8; and

FIG. 12 is an image formed by blending the image of FIG. 10 with theimage of FIG. 11.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Generally, the present invention provides methods for improving FIBmilling endpointing operations. The methods involve generating real-timeimages of the area being milled and real-time images that can beoverlayed to create differential images.

A preferred embodiment of the invention uses software that can createthe differential images and a controller that can process and even acton the results of a differential image. The differential images arecapable of detecting milling that is not occurring uniformly across asample. The differential images may be used in a manual process in thatthat the user can use the detected milling patterns of the differentialimage to correct in subsequent milling.

In another preferred embodiment, the user can use the differentialimages in an automated process. For example, using conventionalautomated tilting and rotating features, a controller can be programmedto recognize the milling patterns and identify instances where themilling patterns are not uniform. In the automated process, thecontroller can adjust the tilt and rotation of the sample stage or theworkpiece in some manner to correct for the non-uniformity.

A differential image is the result of subtraction operations between twoimages. For example, in FIG. 8 and FIG. 9, the corresponding gray levelinformation in each pixel of two images can be subtracted out, whichwould result in a differential image for each pixel. Although theseimages appear very similar, FIG. 9 was collected after a significantamount of milling, using a cleaning cross section. A cleaning crosssection is a type of milling pattern (e.g., milling in a line by lineprogression rather than a box area). A dual beam system operator findsit very difficult to determine what has changed. The resultingdifferential images are shown in FIG. 10 and FIG. 11. FIG. 10 is theresult of subtracting FIG. 8 from FIG. 9, and FIG. 11 is a result ofsubtracting FIG. 9 from FIG. 8. Finally, the two results (FIG. 10 andFIG. 11) can be blended (e.g., “lighten” blend mode in Photoshop) inFIG. 12, to show overall what has changed the most in the sample.

These calculations can be applied in almost real time to show a nearlylive version of differential images on a user interface. This wouldprovide the operator nearly real time feedback on what is milling in hissample and would enable higher quality sample preparation, which wouldfar improve any available technologies to use such high resolutionreal-time data. The differential image may be displayed by itself oroverlayed onto a conventional image to show what has changed betweenmilling passes of the FIB. The differential image could be displayed asa flashing overlay on the last slice. In other embodiments, thesehighlights where the ion beam is milling, that is, where it is hittingthe sample.

In accordance with one embodiment of the current invention, a method forperforming endpointing on a sample is disclosed. To perform theprocedure that includes imaging the sample, the sample is placed in asystem with both a charged particle beam system, such as a focused ionbeam, and an electron beam system. After the sample is loaded in thesystem, the electron beam system is used to create an image on a firstsurface. For purposes of the current invention, the surface to be imagedis a cross sectional slice of the sample. The charged particle beam isused to slice through the sample and create a new sample surface, or asecond surface of the sample. Once again, the electron beam is used toimage the sample on the second surface of the sample, which again isused to create a second cross sectional view of the sample.

FIG. 1 is a figure of diagram of side by side images of SEM imagesacquired during SPI mode spanning more than 20 individually savedimages. Using conventional methods, an operator would have to view theseindividual slices manually by eye to determine minute differences ineach slice to determine the progression of differences in each slice.Once the differences are determined, the operator than can use thedifference to determine the proper region of interest and proceed withthe next milling step. For example, the operator can use the differenceto determine which part of his sample is currently being milled. Thishelps the operator make adjustments to the milling process. This can beperformed by scan rotation, beam shift, or the advancement of the linemilling of a cleaning cross section.

Alternatively, the operator can use the differences in each progressiveslice to follow the progression of milling on the sample. This allowsthe operator to anticipate where sample material is going to be millednext. Understanding where the material sample will be milled next helpsin the preparation for endpointing. It should be understood that inanother preferred embodiment, the automation of this process iscontemplated. If the differences in the milling progressive slices canbe automatically identified, then a controller (not shown) can beprogrammed to make the adjustments in scan rotation, beam shift, oradvancement of the line milling of a cleaning cross-section based on theidentified patterns. Thus, the differential image can be used toidentify the milling patterns, which then allows for the controller tomake those adjustments. Adjustments can be automated in the millingprocess, as well, such as making tilting and rotation adjustments on thesample stage or workpiece (not shown). Conventional controllers andconventional means are known in the art to allow for the automatedcontrols of the sample stage, workpiece, or beam changes.

In accordance with another embodiment of the current invention, softwareis used to create a digital version of the image of the first surfaceand the second surface, and software is additionally used to overlay theimages and create a differential image. The images are compared witheach other and the resultant third image shows the difference made fromthe charged particle beam to create the second surface.

FIG. 2 shows a conventional method of using differential images for TEMsamples wherein the first two samples show progressive slices of a TEMsample and where the third image shows the differential image showingthe differential line 201. As shown, the operator can view thedifferential image and identify the progressive nature of each slice.The line 201 can also be identified by a controller in an automatedprocess using conventional methods, such as reading digital pixels froma scanned image (process not shown).

The method in accordance with the current invention uses the software todetermine color or shade information in each pixel of the images.Various software is capable of comparing images at a pixelated level.Photoshop has a Blend Mode wherein pixels are compared between the firstimage and the second image. The software allows wherein the pixels thatare darker in one image is replaced with pixels that are lighter in themaking of the third image. In this mode, pixels from one image that arethe same in color or shade as the pixels in the second image are leftalone in the third image. This creates a differential image thathighlights the differences in milling patterns made by the millingprocess.

FIG. 3A is a sample TEM slice labeled “n” and FIG. 3B is the next sampleTEM slice labeled “n+1”. The sample 300 has milling sites 301 that arevery difficult to distinguish with the human eye. Similar to thedescription of Exhibit C above, once the image is overlayed and adifferential image is created, the differences in milling is highlightedby image shown in FIG. 3C.

FIG. 3D is the resultant image of the differential after the subtractionis made using the software. As shown, the highlighted differences in 302is much easier to identify with the differential overlayed image.

Much like the exhibits shown above, the differential image can bedisplayed over the last image periodically in a flashing overlay thatcreates highlights wherein the ion beam last milled. This can also beused by a controller to determine the dwell point of the next subsequentmilling process if one is needed.

FIGS. 4A and 4B are cross sectional images 400 of a copper grain TEMsample. Again, in the conventional method, the operator would normallyhave to determine minute differences in these sample surfaces 401A todetermine the next or subsequent milling process.

FIG. 4C is the differential of the cross sectional images of coppergrain TEM samples that again show highlights in the differences made bythe milling processes of the current invention. The method in accordancewith the current invention is used to create many slices or surfaces ofthe sample site using the charged particle beam. Subsequent differentialimages can be created by using the same method and same software tocompare subsequent slices of images.

As described above, a controller can be used to automate the imagecollection of the SEM images and the creation of the differential image.The controller may be used to determine the subsequent dwell points andautomatically mill the subsequent regions of interest on the surfacesusing the highlighted differences from the differential image. Themethods used in these procedures are performed in real time feedback sothat an operator can mill a sample with the FIB immediately followingthe creation of the differential image.

In another embodiment of the current invention, the differential imageof a target structure and its associated acquired image can be definedby the image of low contrast differences in the images. Thus, this is adifferent application than ones that allow the operator to use millingprogressions with a differential image from one slice to the next. Inanother application, the operator can clear a particular layer using FIBmilling (e.g., ILD in an IC circuit) and stop the milling process whenthe next layer is exposed (e.g., SiN or metal layer). The automatedsystem or manual operator can monitor the differential image and detecta contrast change (above a threshold) in a particular part of the image(user definable region of interest). The automated feature, whichrequires significant manipulation at a microscopic level, reduces humanerror and expedites the whole milling process, which stops the millingprocess when a given threshold is met.

To show the extent of multiple slices of an SEM sample, FIG. 5A shows aTEM sample 500 22 nm plan view having slice “n.” FIG. 5B is a TEM sample22 nm plan view having slice “n+1.” FIG. 5C is the overlay process viewwith FIGS. 5A and 5B. FIG. 5D is the overlayed differential viewaccording to an embodiment of the current invention. Again, as shown,the differential image 505 creates easy locations for differences fromthe first two images. FIG. 5E is a TEM sample of a 22 nm plan viewhaving the label slice “n+2.” FIG. 5F is a TEM sample of a 22 nm planview having the label slice “n+3.” FIG. 5G is the overlayed differentialview according to an embodiment of the current invention of FIGS. 5A-5Fwherein the overlay is of a differential image ((n+1)−n) on top of animage (n+1). As shown the contrast allows the operator to clearlyidentify current milling activity of the milling process.

As shown in FIG. 5G, the methods can be reversed so that the first scanimage is overlayed over the second scan image, which creates a negativereverse differential image. Subsequently, these differential images canbe overlayed with each other or with other slices to highlight variousdifferences.

Furthermore, the imaging of the scans are performed using simultaneouspatterning and imaging, which allows the operator to use the FIB toslice away material in an automated slice and view algorithm while atthe same time processing other slices. This allows for a continualprocessing of the slices that result in much higher throughput andefficiency.

Thus, in TEM sample preparation, this process can improve yield, speed,and accuracy. The process facilitates automatic endpointing through theuse of pattern recognition on the differential image. A real image canalso be compared to a calculated image using images derived from CADdata. The image processing can be performed while the next image iscaptured thereby speeding the throughput of processing the images. Theprocess is useful in a variety of applications besides TEM samplepreparation. The process can also be used in tomograph. For example, inviewing biological samples using cryo-tomograph, the system can learnfrom site A what to find at site B, and the system can cease millingafter the region of interest is no longer visible.

FIG. 6 is a flowchart 600 showing an exemplary method of usingdifferential imaging to perform endpointing in a charged particle beamsystem in accordance with embodiments of the present invention. Themethod begins at step 602 and proceeds to step 604. At step 604, thesample is loaded into the charged particle beam system. The chargedparticle beam system includes an ion beam and electron microscope. Atstep 606, the sample is milled, using the ion beam, to expose a firstsurface of the sample. At step 608, an image is formed, using theelectron microscope, of the first surface of the sample. At step 610,the first surface of the sample is milled, using the ion beam, to exposea second surface of the sample. At step 612, a second image of thesecond surface of the sample is formed using the electron microscope. Atstep 614, a third image is formed by overlaying the second image overthe first image. The third image is a differential image formed bysubtracting the second image from the first image. The third image showsthe difference made from the ion beam milling to create the secondsurface.

FIG. 7 depicts of an exemplary dual beam SEM/FIB system 702 that isequipped to form samples and move them to a TEM grid. Suitable dual beamsystems are commercially available, for example, from FEI Company,Hillsboro, Oreg., the assignee of the present application. While anexample of suitable hardware is provided below, the invention is notlimited to being implemented in any particular type of hardware.

Dual beam system 702 has a vertically mounted electron beam column 704and a focused ion beam (FIB) column 706 mounted at an angle ofapproximately 52 degrees from the vertical on an evacuable specimenchamber 708. The specimen chamber may be evacuated by pump system 709,which typically includes one or more, or a combination of, aturbo-molecular pump, oil diffusion pumps, ion getter pumps, scrollpumps, or other known pumping means.

The electron beam column 704 includes an electron source 710, such as aSchottky emitter or a cold field emitter, for producing electrons, andelectron-optical lenses 712 and 714 forming a finely focused beam ofelectrons 716. Electron source 710 is typically maintained at anelectrical potential of between 500 V and 30 kV above the electricalpotential of a work piece 718, which is typically maintained at groundpotential.

Thus, electrons impact the work piece 718 at landing energies ofapproximately 500 eV to 30 keV. A negative electrical potential can beapplied to the work piece to reduce the landing energy of the electrons,which reduces the interaction volume of the electrons with the workpiece surface, thereby reducing the size of the nucleation site. Workpiece 718 may comprise, for example, a semiconductor device,microelectromechanical system (MEMS), data storage device, or a sampleof material being analyzed for its material characteristics orcomposition. The impact point of the beam of electrons 716 can bepositioned on and scanned over the surface of a work piece 718 by meansof deflection coils 720. Operation of lenses 712 and 714 and deflectioncoils 720 is controlled by scanning electron microscope power supply andcontrol unit 722. Lenses and deflection unit may use electric fields,magnetic fields, or a combination thereof.

Work piece 718 is on movable stage 724 within specimen chamber 708.Stage 724 can preferably move in a horizontal plane (X-axis and Y-axis)and vertically (Z-axis) and can tilt approximately sixty (60) degreesand rotate about the Z-axis. A door 727 can be opened for inserting workpiece 718 onto X-Y-Z stage 724 and also for servicing an internal gassupply reservoir (not shown), if one is used. The door is interlocked sothat it cannot be opened if specimen chamber 708 is evacuated.

Mounted on the vacuum chamber are one or more gas injection systems(GIS) 730. Each GIS may comprise a reservoir (not shown) for holding theprecursor or activation materials and a needle 732 for directing the gasto the surface of the work piece. Each GIS further comprises means 734for regulating the supply of precursor material to the work piece. Inthis example the regulating means are depicted as an adjustable valve,but the regulating means could also comprise, for example, a regulatedheater for heating the precursor material to control its vapor pressure.

When the electrons in the electron beam 716 strike work piece 718,secondary electrons, backscattered electrons, and Auger electrons areemitted and can be detected to form an image or to determine informationabout the work piece. Secondary electrons, for example, are detected bysecondary electron detector 736, such as an Everhart-Thornley detector,or a semiconductor detector device capable of detecting low energyelectrons. Signals from the detector 736 are provided to a systemcontroller 738. Said controller 738 also controls the deflector signals,lenses, electron source, GIS, stage and pump, and other items of theinstrument. Monitor 740 is used to display user controls and an image ofthe work piece using the signal

The chamber 708 is evacuated by pump system 709 under the control ofvacuum controller 741. The vacuum system provides within chamber 708 avacuum of approximately 7×10-6 mbar. When a suitable precursor oractivator gas is introduced onto the sample surface, the chamberbackground pressure may rise, typically to about 5×10-5 mbar.

Focused ion beam column 706 comprises an upper neck portion 744 withinwhich are located an ion source 746 and a focusing column 748 includingextractor electrode 750 and an electrostatic optical system including anobjective lens 751. Ion source 746 may comprise a liquid metal galliumion source, a plasma ion source, a liquid metal alloy source, or anyother type of ion source. The axis of focusing column 748 is tilted 52degrees from the axis of the electron column. An ion beam 752 passesfrom ion source 746 through focusing column 748 and betweenelectrostatic deflectors 754 toward work piece 718.

FIB power supply and control unit 756 provides an electrical potentialat ion source 746. Ion source 746 is typically maintained at anelectrical potential of between 1 kV and 60 kV above the electricalpotential of the work piece, which is typically maintained at groundpotential. Thus, ions impact the work piece at landing energies ofapproximately 1 keV to 60 keV. FIB power supply and control unit 756 iscoupled to deflection plates 754 which can cause the ion beam to traceout a corresponding pattern on the upper surface of work piece 718. Insome systems, the deflection plates are placed before the final lens, asis well known in the art. Beam blanking electrodes (not shown) withinion beam focusing column 748 cause ion beam 752 to impact onto blankingaperture (not shown) instead of work piece 718 when a FIB power supplyand control unit 756 applies a blanking voltage to the blankingelectrode.

The ion source 746 typically provides a beam of singly charged positivegallium ions that can be focused into a sub one-tenth micrometer widebeam at work piece 718 for modifying the work piece 718 by ion milling,enhanced etch, material deposition, or for imaging the work piece 718.

A micromanipulator 757, such as the AutoProbe 200™ from Omniprobe, Inc.,Dallas, Tex., or the Model MM3A from Kleindiek Nanotechnik, Reutlingen,Germany, can precisely move objects within the vacuum chamber.Micromanipulator 757 may comprise precision electric motors 758positioned outside the vacuum chamber to provide X, Y, Z, and thetacontrol of a portion 759 positioned within the vacuum chamber. Themicromanipulator 757 can be fitted with different end effectors formanipulating small objects. In the embodiments described herein, the endeffector is a thin probe 760. A micromanipulator (or microprobe) can beused to transfer a TEM sample (which has been freed from a substrate,typically by an ion beam) to a TEM grid in a TEM sample holder 761 foranalysis. Stage 724 can also include mounted thereon a flip stage (notshown) as described for example in U.S. Pat. Pub. No. 20040144924 ofAsselbergs et al. for “Method for the Manufacture and TransmissiveIrradiation of a Sample, and Particle-optical System,” which is owned bythe applicant of the present invention and which is hereby incorporatedby reference. Mounting the TEM grid on the flip stage allows theorientation of the TEM grid to be changed and, with rotation of thestage, allows the sample can be mounted in a desired orientation.

System controller 738 controls the operations of the various parts ofdual beam system 702. Through system controller 738, a user can causeion beam 752 or electron beam 716 to be scanned in a desired mannerthrough commands entered into a conventional user interface (not shown).Alternatively, system controller 738 may control dual beam system 702 inaccordance with programmed instructions. FIG. 7 is a schematicrepresentation, which does not include all the elements of a typicaldual beam system and which does not reflect the actual appearance andsize of, or the relationship between, all the elements.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. For example,the use of overlayed differential images of TEM samples can be used inthe field of tomography, which includes automated diffractiontomography. This method is known from “Towards automated diffractiontomography: Part I—Data acquisition”, U. Kolb et al., Ultramicroscopy107 (2007) 507-513. The teachings of the embodiments of the currentinvention can be applied to many different arts, including the use ofstudying bio samples. It can also be used in the field ofcryo-tomography, wherein a system can be provided to look for regions ofinterest from site A and use the findings to teach a system to find theregions of interest in site B. Tomography can be used to stop millingsequences after the region of interest is removed from a sample site.The invention can also be applied to IR CE applications and subsurfaceimaging.

I claim:
 1. A method for performing endpointing on a sample with acharged particle beam system comprising: loading a sample into a chargedparticle beam system, the charged particle beam system including an ionbeam and electron microscope; milling, using the ion beam, the sample toexpose a first surface of the sample; forming, using the electronmicroscope, a first image of the first surface of the sample; milling,using the ion beam, the first surface of the sample to expose a secondsurface of the sample; forming, using the electron microscope, a secondimage of the second surface of the sample; forming a third image byoverlaying the second image over the first image, the third image beinga differential image formed by subtracting the second image from thefirst image, the third image showing the difference made from the ionbeam milling to create the second surface; wherein creating the thirdimage includes determining whether a grayscale level of one or morepixels of the first image is darker than the grayscale level of thecorresponding pixel of the second image; and forming a fourth image byoverlaying the first image over the second image, the fourth image beinga differential image formed by subtracting the first image from thesecond image, the fourth image showing the difference made from the ionbeam milling to create the second surface.
 2. The method in accordancewith claim 1 further comprising forming a fifth image by replacingpixels that are darker in the third image with the corresponding pixelsthat are lighter in the fourth image and replacing pixels that aredarker in the fourth image with the corresponding pixels that arelighter in the third image.
 3. The method in accordance with claim 2where the third image, the fourth image, or the fifth image can bedisplayed periodically in a flashing overlay over second image therebycreating highlights showing where the ion beam last milled.
 4. Themethod in accordance with claim 2 further including: milling, using theion beam, the second surface of the sample to create a third surface ofthe sample; imaging the third surface of the sample using the electronmicroscope thereby creating a sixth image of the sample; and overlayingthe sixth image over the second image creating a differential image thatshows the difference made from the charged particle beam to create thethird surface.
 5. The method in accordance with claim 2 wherein acontroller is used to automate the image collection of the first andsecond image and the creation of the third differential image, thecontroller including a computer processor and a computer-readablememory.
 6. The method in accordance with claim 5 wherein said controlleruses the third differential image to automatically mill the secondsurface based on the differences from the differential image.
 7. Themethod in accordance with claim 6 wherein the dwell point of the ionbeam on a surface of the sample can be determined by the controllerusing the third differential image.
 8. The method in accordance withclaim 1 further comprising performing calculations to create adifferential image substantially in real time feedback so that anoperator of the charged particle beam system can mill a sample with theion beam immediately following the creation of the differential image.9. A method for performing endpointing on a sample with a chargedparticle beam system comprising: loading a sample into a chargedparticle beam system, the charged particle beam system including an ionbeam and electron microscope; milling, using the ion beam, the sample toexpose a first surface of the sample; forming, using the electronmicroscope, a first image of the first surface of the sample; milling,using the ion beam, the first surface of the sample to expose a secondsurface of the sample; forming, using the electron microscope, a secondimage of the second surface of the sample; forming a third image byoverlaying the first image over the second image, the third image beinga differential image formed by subtracting the first image from thesecond image, the third image showing the difference made from the ionbeam milling to create the second surface; wherein creating the thirdimage includes determining whether a grayscale level of one or morepixels of the second image is darker than the grayscale level of thecorresponding pixel of the first image; and forming a fourth image byoverlaying the second image over the first image, the fourth image beinga differential image formed by subtracting the second image from thefirst image, the fourth image showing the difference made from the ionbeam milling to create the second surface.
 10. The method in accordancewith claim 9 further comprising a fifth image by replacing pixels thatare darker in the first image with the corresponding pixels that arelighter in the second image and replacing pixels that are darker in thesecond image are replaced with the corresponding pixels that are lighterin the second image.
 11. The method in accordance with claim 10 whereinthe third image can be displayed periodically in a flashing overlay oversecond image thereby creating highlights showing where the ion beam lastmilled.
 12. The method in accordance with claim 10 further including:milling, using the ion beam, the second surface of the sample to createa third surface of the sample; imaging the third surface of the sampleusing the electron microscope thereby creating a sixth image of thesample; and overlaying the sixth image over the second image creating adifferential image that shows the difference made from the chargedparticle beam to create the third surface.
 13. The method in accordancewith claim 10 wherein a controller is used to automate the imagecollection of the first and second image and the creation of the thirddifferential image, the controller including a computer processor and acomputer-readable memory.
 14. The method in accordance with claim 13wherein said controller uses the third differential image toautomatically mill the second surface based on the differences from thedifferential image.
 15. The method in accordance with claim 14 whereinthe dwell point of the ion beam on a surface of the sample can bedetermined by the controller using the third differential image.
 16. Themethod in accordance with claim 9 further comprising performingcalculations to create a differential image substantially in real timefeedback so that an operator of the charged particle beam system canmill a sample with the ion beam immediately following the creation ofthe differential image.